Photocatalytic toluene oxidation with nickel-mediated cascaded active units over Ni/Bi2WO6 monolayers

Adsorption and activation of C–H bonds by photocatalysts are crucial for the efficient conversion of C–H bonds to produce high-value chemicals. Nevertheless, the delivery of surface-active oxygen species for C–H bond oxygenation inevitably needs to overcome obstacles due to the separated active centers, which suppresses the catalytic efficiency. Herein, Ni dopants are introduced into a monolayer Bi2WO6 to create cascaded active units consisting of unsaturated W atoms and Bi/O frustrated Lewis pairs. Experimental characterizations and density functional theory calculations reveal that these special sites can establish an efficient and controllable C–H bond oxidation process. The activated oxygen species on unsaturated W are readily transferred to the Bi/O sites for C–H bond oxygenation. The catalyst with a Ni mass fraction of 1.8% exhibits excellent toluene conversion rates and high selectivity towards benzaldehyde. This study presents a fascinating strategy for toluene oxidation through the design of efficient cascaded active units.

a 300 W Xenon lamp (Beijing Perfectlight Technology Co. Ltd., PLS-SXE300D) for 4 h.The result product was washed with deionized water and dried at 60 ℃ for 24 h.The obtained powder was further heated at 200 ℃ in O2 atmosphere.
The final sample was named as Ni/BWO-surface.

Preparation of BiOCl nanosheets with O vacancy (BiOCl)
BiOCl was prepared via the method reported in our previous studies. 1002 mol of Bi(NO3)3•5 H2O and 0.800 g of PVPk30 were added into 50 mL of 0.1 M mannitol solution under stirring.Then, 10 mL of saturated NaCl solution was added into the mixed solution.The mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave after stirring for 10 min and heated at 160 ℃ for 5 h.The precipitates were washed with ethanol and deionized water.
Finally, the products were dried at 60 ℃ for 6 h.

In situ DRIFT and EPR test
The FT-IR spectra of the powder samples and the time-dependent in situ diffuse reflectance infrared Fourier-transform (DRIFT) spectra were recorded on a NICOLET IS50 Fourier transform infrared (FT-IR) spectrometer at a resolution of 4 cm -1 , undergoing a total of 64 scans.Before collecting the in situ DRIFTS spectra, the samples were treated at 150 ℃ for 2 h in N2 atmosphere.
Then pure O2 (99.99%) was continuously introduced into the sample chamber.
The time-dependent in situ DRIFT spectra for adsorbing O2 in the dark were recorded.For the in-situ characterization of the photocatalytic toluene oxidation, pure O2 (99.99%) saturated with toluene vapor was continuously introduced into the sample chamber.The time-dependent in situ DRIFT spectra for adsorbing toluene in the dark were recorded.Next, the sample was irradiated using a 300-W Xe lamp.The time-dependent in situ DRIFT spectra under visible light irradiation were recorded.
EPR spectra of the powder samples were recorded on a Brucker A300 spectrometer.The in situ EPR spectra for detecting carbon central radical were carried out under visible-lightirradiation (λ> 400 nm). 10 mg of the prepared sample, 0.1 mmol of N-tert-Butyl-α-phenylnitrone (PBN), 2 mL of toluene were mixed in a quartz reactor.N2 gas was passed through the solution to remove O2.Then, the mixture was collected by a capillary and transferred to a closed EPR tube with N2 and then the in situ EPR spectra of the samples were recorded.The in situ EPR spectra for detecting •O2 -radical were carried out under the same method, but the trapping agent was 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and the atmosphere was O2.

Apparent quantum efficiency of the conversion of TL and turnover frequency
The apparent quantum efficiency (AQE) for the conversion of TL was measured using a 300 W Xenon lamp with a 420 nm band pass filter. 2 The total irradiance was 15.0 mW cm -2 .The irradiation area was controlled as 4 cm 2 .
Depending on the amount of converted TL by the photocatalytic reaction in 6 hours, and the AQY was calculated as follow: Where, M is the molar mumber of transformed TL (mol), NA represents Avogadro constant (6.022×1023 mol -1 ), ℏ represents the Planck constant (6.626×10 -34 J•s), c represents the speed of light (3×10 8 m s -1 ), S represents the irradiation area (cm 2 ), P represents the intensity of irradiation light (W cm - 2 ), t represents the reaction time (s), λ represents the wavelength of the monochromatic light (nm).
Turnover frequency (TOF) is calculated according to the formulas in previous study using the moles of the catalyst.SEM images of these samples are shown in Supplementary Fig. 12. BWObulk shows a morphology of bulk accumulation (Supplementary Fig. 12a).This morphology is disadvantageous for the exposure of surface active sites.After the heat treatment of BWO nanosheet, the nanosheet structure is still remained.
Similarly, Ni/BWO-surface and BiOCl also exhibit the nanosheet structure.The morphologies of these samples are well controlled, which is convenient to study stronger than the W-O interaction in adsorbed oxygen species, making the migration process of lattice O may be more difficult.The activation energy for lattice O (6-coordinated O with W) to migrate to a 5-coordinated Ni atom was calculated to be 2.44 eV (with a reaction energy of 1.91 eV).(Supplementary Fig. 23c) This is 2.1 eV higher than the oxygen migration barrier in adsorbed oxygen species.Therefore, the 16  In addition, the free radical quenching experiments further explain the role of these active species (Supplementary Fig. 24c).When NH4COOH is used to quench photogenerated h + , the conversion rate of TL is significantly reduced to 243 μmol g -1 h -1 , indicating that the deprotonation of TL by photogenerated holes is the key step for the oxidation of TL.Due to the similar valence band potential of BWO (2.49V vs. NHE) and 1.8 Ni/BWO (2.39 V vs. NHE) (Supplementary Fig. 13), they all meet the oxidation potential of TL (2.18 V vs. NHE).However, BWO produces the weakest carbon-centered radical signal.
This reflects the importance of the adsorption and activation of TL on the FLPs.
Moreover, there is no TL to be converted without O2, as the oxidation of TL requires O2 as an oxygen source.When the photogenerated electrons and •O2 - radicals are quenched by K2S2O7 and BQ, the conversion rate of TL is 1655 and 1921 μmol g -1 h -1 , respectively.This result indicates that •O2 -radicals are considered as one of the active O species in the subsequent oxidation process and another O transfer pathway also exists.
products such as carbon center radicals (*C7H7), benzyl alcohol step by step.
Due to the unique surface active structure of 1.8 Ni/BWO, the main product is benzaldehyde.As the accumulation of benzaldehyde, it can be further oxidized to benzoic acid by •O2 -species. 6,14 his is consistent with our experiment results of toluene oxidation.
Supplementary Table 2 The fraction of W 5+ and W 6+ in the prepared samples.The experiments for the photocatalytic oxidation of toluene derivatives are conducted.Table S6 show that the toluene derivatives with electron-donating group (-NH2) are easier to convert to the product than those with electronwithdrawing groups (-F, -Cl, -NO2).The selectivity of the corresponding benzaldehyde derivatives is reduced due to the removal of other substitutions.
The result shows that 1.8 Ni/BWO exhibits good conversion of other toluene derivatives.
Compared with the values of k (υC-H) on KBr, two C-H bonds are obviously weakened while one C-H bond is strengthened, 42 indicating that the products on FLPs may be easily converted to BD.This is consistent with the result of photocatalytic activity results.The changes in k (υC-H) values on BWO are small due to the absence of FLPs.The interaction between FLPs and TL would provide bridges for the migration of photogenerated holes from the catalyst to the -CH3 group of toluene and the activated C-H bonds are easily broken and dehydrogenated.
Supplementary Table 10 The Bader charge of spatially separated Bi and O atoms in

3 TOF=
moles of consumed toluene / (moles of catalyst × reaction time).surface of BWO nanosheet without the formation of Ov and unsaturated W to study the influence of Ni.In addition, BiOCl with Ov is conducted to illustrate the influence of Ov and unsaturated Bi atoms.As shown in Supplementary Fig. 11, XRD peaks of all the prepared BWO-bulk, BWO-Ov and Ni/BWO-surface samples are well matched with orthorhombic Bi2WO6.The peak intensity of BWO-bulk is the strongest, indicating its good crystallinity.After the heat treatment, the peak intensity of BWO-Ov is the lowest, which forebodes the formation of Ov.Moreover, after loading Ni on the surface of BWO, no other peaks are observed.This may be due to the small content of Ni.The XRD pattern of BiOCl is also well match with the with the published data (JCPDS No. 6-0249), 1 confirming its successful preparation.Supplementary Fig. 17 SEM images of the prepared BWO-bulk (a), BWO-Ov (b), Ni/BWO-surface (c) and BiOCl (d).
photogenerated holes.Meanwhile, when using 5,5-dimethyl-1-pyrroline-Noxide (DMPO) as a trapping agent, •O2 -radical signals are detected (Supplementary Fig. 24b).No signals are detected in the dark, indicating the necessary condition of the visible light irradiation.1.8 Ni/BWO shows the strongest EPR signal which is consistent with its best performance in TL oxidation.One reason for this is that the OV favors the separation of photogenerated carriers (Supplementary Fig. 25).Another important reason is that the unsaturated W atoms and FLPs in 1.8 Ni/BWO enhance the activation Ni/BWO-(001) and Ni-OH/BWO-(001) structure modes.

Table 3
The fraction of O species in the prepared samples.

Table 4
The fitting results of W L3-edge XAFS spectra.

Table 5
Apparent quantum efficiency (AQE) of the prepared samples for toluene oxidation.